The present invention relates broadly to bulk material supply systems and, more particularly, to a feed system for bulk solids and its associated controller wherein the controller employs fuzzy logic for rapid reactions to changes in weight.
Bulk solids appear throughout industry and must be handled or processed in a manner which will provide consistent results without waste. Examples of industries which utilize bulk solids include food, lumber, paper, chemical, petroleum refining, rubber, stone, clay, glass, and concrete. The bulk solid materials may include such diverse items as cabbage flakes, bleach, bauxite, baking soda, fiberglass, flour, grass seed, iron ore, starch, sugar, raisins, parsley, noodles, nylon, mineral fiber, mica, lime, and detergent. All of these materials share a common feature in that they are flowable solids. They are fungible materials which may be poured for filling into containers or otherwise metered under flow conditions for processing.
Typically, such processing includes providing a hopper filled with the bulk material with some form of regulator or metering device providing the necessary flow control for processing.
One method of determining how much bulk solid material has been dispensed during any given dispensing operation is by using the so-called loss-in-weight method. There, the hopper containing the bulk material is continually weighed and the reduction in weight is indicative of the material dispensed.
Further, the reduction in weight-per-unit time is indicative of the rate at which a predetermined amount of the bulk material is dispensed. Therefore, by precisely determining the change in weight of the hopper, a precise indication of the amount of material dispensed is realized. This information can be used for controlling the feed process. Furthermore, while the present invention is described for operation with a vibratory feeder, it should be noted that it is equally effective with screw feeders or belt feeders. The focus of the present invention lies in its applicability to loss-in-weight feeders.
A common method which is used to control dispensing of bulk solids is the proportional method used throughout the industry. The setpoint for such control method is a desired rate of change of weight for a supply of the bulk solid. However, because the control variable (the weight change) is the integral of the rate of weight change and not the rate of weight change itself, the proportional control method currently used in the industry, while reliable, is clumsy and lacks a certain amount of precision.
Several methods may be used to control the actual feed of the bulk solid material. Valves may be used which are open proportionally or open for a certain time period. Other methods include the use of a belt feeder or a screw feeder. A vibratory feeder is disclosed in Peschl U.S. Pat. No. 3,973,703. There, a vibrating tray is disclosed containing a number of plates in an angular relationship to one another. According to this method, the bulk material will not flow unless the feeder is vibrated or otherwise agitated in some manner. Such feeders allow for precise flow with the flow rate being based on the amplitude of vibration or agitation. This type of feeder can provide accurate flow characteristics and can respond rapidly to changing inputs from an associated excitation motor. The motor is driven responsive to a control variable. As discussed above, the current proportional controllers are slow to respond to changes in demand or process changes and, therefore, the precision and rapid changing characteristics of any loss in weight type feeders including the vibratory feeder are not fully realized using a proportional controller.